As I said Harry has watched waaayy too many movies with a British villain, so a telepathic voice from a previously unknown villain is automatically processed through his language centre of his brain as British, that part of his brain is stuck that way. The British Prisoner is not necessarily British as a consequence, he only sounds it, because of Harry’s brain damage. He or she could be anyone.It doesn’t help that Binder actually sounds like the late Bob Hoskins, it reinforces Harry’s brain damage delusion to have an actual villain conform to the stereotype.
I would keep an eye on The Merlin, whose telepathic voice in Turncoat, yes, included a British accent. I bet everyone else heard it in their native language and accent, or Latin.
On this basis the “British” Prisoner could be anyone and the accent is a red herring.
For all we know it is actually Oberon, and this was the only way to get out of the Mab/Titania love triangle unscathed. Hell hath no fury like a woman scorned and all that.